Photolithographically patterned micro spring structures (sometimes referred to as “microsprings”) represent one form of MEMS devices that have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical micro spring structure includes a spring finger having an anchor portion secured to a substrate or other supporting structure, and a free (cantilevered) portion extending from the anchored portion over the substrate. The spring finger is formed from a stress-engineered film (i.e., a film fabricated such that portions closer to the underlying substrate have a higher internal compressive stress than portions located farther from the substrate) that is at least partially formed on a release material layer. The free portion of the spring finger bends away from the substrate when the release material located under the free portion is etched away, thereby “releasing” the spring finger portion from the substrate. The internal stress gradient is produced in the spring by layering, e.g., different metals having the desired stress characteristics, or by using a single metal by altering the fabrication parameters. Such spring structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
Recent developments related to micro spring-type MEMS devices include the ability to actuate (i.e., control the position of) a tip portion of a released spring finger by selectively applying an electrostatic actuating force to pull the cantilevered free portion toward the underlying substrate (i.e., against the bending force generated by the internal stress gradient of the spring finger). Electrostatic actuation is currently considered the most energy efficient method of actuating MEMS devices due to physical scaling laws. Electrostatically actuated MEMS devices utilize an electrode that is located adjacent to (e.g., on the substrate directly under) the free end of the spring finger, and associated driver circuit that applies a suitable voltage onto the electrode. The thus-charged electrode attracts the movable (e.g., free) portion of the MEMS device, causing this movable portion to bend toward the electrode against the biasing spring force produced by its internal stress gradient. By controlling the voltage level on the electrode, the position of the MEMS device relative to the supporting substrate can be altered between a fully deployed position (i.e., where the electrode voltage is minimized and the MEMS device is fully biased away from the substrate) and a fully retracted position (i.e., where the electrode voltage is maximized and the MEMS device is pulled against the electrode/substrate).
A problem associated with electrostatically actuated MEMS devices is that, although the actuating stroke length is generally proportional to the applied voltage, the maximum force is usually proportional to the voltage squared. Therefore, in practical applications, a high voltage (i.e., in excess of 50 Volts, often 100 V or more) is needed to drive these electrostatically actuated MEMS devices. Supplying this high voltage to the electrode of an electrostatically actuated MEMS device is not a major concern because many efficient DC—DC conversion power supplies are readily available, especially for the low current requirement of most electrostatically actuated MEMS devices. Controlling the high voltage, however, is not straight forward because such high voltages are not compatible with standard CMOS ULSI technology. That is, standard CMOS ULSI technology produces circuit structures having operating voltages in the range of about 0 V to 20 V. To address this high voltage control problem, special high voltage power chips are currently used to interface between the control circuitry and the actuated MEMS devices. Although such external power chips can be purchased “off the shelf” (i.e., relatively inexpensively), the use of external power chips requires a separate connection for each MEMS device. Accordingly, it would be very difficult and expensive to produce large MEMS device arrays using external power chips because the number of required interconnects increases as the square of array size, which increases both manufacturing time and cost.
What is needed is a method and structure that integrates electrostatically-actuated MEMS devices and high-voltage driver circuits with low voltage control circuitry on a single substrate, thereby facilitating the production of large electrostatically-actuated MEMS device arrays.